FIG. 12 is a configuration view showing an example of a conventional gas laser device, and illustrates a tri-axially orthogonal CO2 laser oscillator described in Patent Document 1. A laser gas is enclosed at a pressure of about tens Torrs inside the device. The laser gas is supplied into a discharge space 51 in the direction indicated by an arrow by means of a blower 52. When silent discharge occurs in the discharge space 51, CO2 molecules are excited, so that the stimulated emission produces laser light in the direction perpendicular to the sheet. Here, a reflector 54 of a laser resonator is located so that a laser optical axis coincides with the gas downstream end of electrodes. The laser gas which has passed through the discharge space 51 is cooled in a heat exchanger 53.
FIG. 13 shows a relationship between a gain distribution and a position of discharge electrode in the tri-axially orthogonal CO2 laser oscillator, which is described in detail in Non-Patent Document 1. When silent discharge occurs between cylindrical electrodes whose surfaces are coated with glass, the gain distribution is gradually increased from the gas upstream end of the electrodes, peaks at the gas downstream end thereof, and is gradually decreased along the gas downstream side. At this time, the optical axis of the resonator coincides with the vicinity of the gas downstream end of the electrodes at which the gain distribution peaks.
Such a gain distribution can be expressed by an exponential function, as shown by the following equation. Here, XD is an width of electrode, λ is a relaxation rate at a higher level of laser, υ is a flow rate of laser gas, σ is a cross section of stimulated emission, η is an excitation efficiency, w is a discharge power density, and X is a coordinate in direction of gas flow.
                              [                      Equation            ⁢                                                  ⁢            1                    ]                ⁢                                                                                                            g            0                    ⁡                      (            X            )                          =                                                                              ση                  ⁢                                                                          ⁢                  w                                λ                            ⁡                              [                                  1                  -                                      exp                    ⁡                                          (                                                                        -                          λ                                                ⁢                                                                                                  ⁢                                                  X                          /                          v                                                                    )                                                                      ]                                      ⁢                                                  ⁢            0                    <          X          <                      X            D                                              (        7        )                                                                    g              0                        ⁡                          (              X              )                                =                                                                      ση                  ⁢                                                                          ⁢                  w                                λ                            ⁡                              [                                                      exp                    ⁡                                          (                                              λ                        ⁢                                                                                                  ⁢                                                                              X                            D                                                    /                          v                                                                    )                                                        -                  1                                ]                                      ×                          exp              ⁡                              (                                                      -                    λ                                    ⁢                                                                          ⁢                                      X                    /                    V                                                  )                                                    ⁢                                  ⁢                              X            D                    <          X                                    (        8        )            
FIGS. 14A, 14B, and 14C show an example of asymmetrical beam mode distribution: FIG. 14A shows contour lines of a beam intensity distribution, in which the arrow indicates the direction of laser gas flow, and FIG. 14B shows the intensity distribution of a center cross-section in the horizontal direction, and FIG. 14C shows the intensity distribution of a center cross-section in the vertical direction. In the conventional gas laser device as disclosed in Patent Document 1 and Non-Patent Document 1, the optical axis of the resonator is set at the peak position of the gain distribution so that the highest oscillation efficiency is achieved.
In the tri-axially orthogonal laser oscillator, as shown in FIG. 13, the gain distribution shows such an intensity distribution changed in the gas flow direction due to the presence of the gas flow. On the other hand, there is no gas flow in the direction of discharge gap length, exhibiting a substantially uniform gain distribution. That is, the gain distribution is different between in the gap length direction and in the gas flow direction. As shown in FIGS. 14A, 14B, and 14C, due to such anisotropy of the gain distribution, the intensity distribution of the outputted beam is asymmetrical between in the gap length direction and in the gas flow direction.
The discharge-excited laser, such as CO2 laser, can produce a higher output as the discharge power is further increased. However, when the discharge power is too large, an arc discharge occurs so that the discharge is likely to be unstable. To solve this problem, the conventional tri-axially orthogonal laser oscillator adopts discharge electrodes each having a relatively large width of electrode so that the discharge power density cannot be too high. Therefore, the anisotropy of the gain distribution is not so great, so that the asymmetry of the intensity distribution of the laser beam does not matter much.
In recent years, the discharge control technique is improved so that stable discharge can be achieved even when the electrode width is reduced to increase the discharge power density. In addition, to increase the efficiency of the tri-axially orthogonal laser oscillator, it is effective to reduce the electrode width, but the anisotropy of the gain distribution is likely to occur. When the anisotropy of the gain distribution is great, the intensity distribution of the outputted laser beam is likely to be asymmetrical, as shown in FIGS. 14A, 14B, and 14C. In case such an asymmetrical laser beam is used for cutting, the anisotropy occurs on the cut surface of a workpiece, resulting in deteriorated cutting quality.
FIG. 15 is a plan view showing another example of a conventional tri-axially orthogonal CO2 laser oscillator. FIG. 16 is a transverse sectional view of discharge electrodes. These drawings are described in Patent Document 2. A laser gas is supplied to a discharge region 66 located between electrodes 61A and 61B and between electrodes 62A and 62B. A rear mirror 63 and an output mirror 64 of an optical resonator are opposite to each other so as to flow the laser gas therebetween. A laser beam LB is amplified in an optical cavity 65 defined by the rear mirror 63 and the output mirror 64. Then, a part of amplified laser beam LB is outputted from the output mirror 64. Here, two sets of electrodes 61A and 61B and electrodes 62A and 62B are each shifted in the gas flow direction so as to be located in different positions with respect to the optical cavity 65, thereby achieving a uniform gain distribution.
FIG. 17 is a graph showing a relationship between the gain of the laser gas excited when it passes through the optical cavity 65 and the position in the optical path of the resonator. This exemplifies a case wherein three sets of discharge electrodes are each shifted to different positions with respect to the resonator's optical path. The laser gas excited by the first discharge electrodes in the position farthest from the resonator's optical path shows a gain curve 67 having a peak P1 on the gas upstream side. The laser gas excited by the third discharge electrodes in the position closest to the resonator's optical path shows a gain curve 69 having a peak P3 on the gas downstream side. The laser gas excited by the second discharge electrodes located between the first discharge electrodes and the third discharge electrodes shows a gain curve 68 having a peak P2 between the peaks P1 and P3. Therefore, the overlap of the three gain curves 67 to 69 can obtain a gain curve 70 showing a substantially uniform gain distribution in the cross section of the resonator's optical path.